Macrophages and T cells interact with each other and in the process synthesize and secrete a variety of antigen-nonspecific soluble factors which serve to induce andamplify both humoral and cell-mediated immune responses. We have utilized our basic observations on the effects of phorbol esters on lymphocytes to describe a technique to induce mouse spleen cells to produce 5-20 times as much Interleukin 2 (IL2) as conventionally stimulated spleen cells. In addition, the phorbol esters have been used to stimulate the murine thymoma, EL-4 to produce IL 2 in very large quantities in vitro. The use of the thymoma culture supernatants as a source of IL 2 should facilitate the purification to homogeneity of the factor. Additionally, the large amounts of factor obtainable from the thymoma should allow in vivo studies of possible therapeutic uses for IL 2. Studies on the mechanism of action of IL 2 indicate that the factor interacts with a receptor on the target cell and induces that cell to produce immune interferon which is required for the subsequent lymphocyte activation events.